1. Field of the Invention
The present invention relates to a process and a device for analysing interference in a cellular radiocommunication system of the GSM or DCS 1800 type for example.
2. Discussion of the Background
In a cellular radiocommunication system the coverage of a specified territory is achieved using cells served by base stations and by applying a pattern for reusing frequencies which makes it possible to increase the capacity of the network.
Various types of cells are used. These types consist of macrocells and microcells. A macrocell employs a base station placed on a high point such as the roof of a building and its radius may reach several kilometres. A microcell serves areas of smaller radius of a few hundred metres. It employs a base ministation whose aerial is generally located below the roof of buildings, on lampposts for example.
In these systems the considerable reuse of frequencies, in particular in areas of high subscriber density, gives rise to considerable co-channel interference and interference on adjacent channels. Scheduling of the network is therefore necessary to eliminate this interference. This scheduling is carried out by an operator who has available a number of software and hardware tools enabling him to assess the match between the population of subscribers to be served and the capacity of the network installed.
The software utility allows the operator to plan the siting of the base stations and their characteristics as a function of the population of subscribers to be served.
The hardware utility is transported on vehicles. It comprises a trace function allowing storage of the measurements made on the network.
Within the context of a GSM network, the main problem of interference arises in relation to the downlink between base station and mobile since the base stations are generally placed on high points which are ideal for transmitting signals over long distances. The engineering of the network generally provides for a xe2x80x9ctiltxe2x80x9d, that is to say an inclination of the antennas so as to force the radiation from the base station to cover only the territory inside its cell. However, this xe2x80x9ctiltxe2x80x9d is not always sufficient to avoid untimely radiations to adjacent cells which are the cause of interference between the transmissions from the various base stations.
The problem is then for the operator to identify the origin of the interference detected in the neighbouring cells. This detection is ordinarily performed with the aid of relatively unsophisticated trace mobiles whose equipment is similar to that with which the customary communication mobiles are equipped. These trace mobiles provide the operator with the information used by the customary mobiles to qualify a communication, that is to say the information RX LEVEL and RX QUAL. This information, whose mode of obtainment is described in the GSM standard, makes it possible to qualify the level of signal received (RX LEVEL) as well as the quality of the communication (RX QUAL). Under these conditions a situation of interference is detected when the level of the signal received is high whilst the quality of the communication is declared to be poor. The problem for the operator is then to identify the jammer while the trace mobile is not providing any identifier or marker therefor.
Furthermore, the software scheduling tools are incapable of handling local features such as for example an opening in a row of buildings which favours undesirable radiations.
The procedure followed by the operator calls greatly on his experience. It consists, by deduction with the aid of the scheduling data base, in identifying the disturbing station by disconnecting it momentarily from the network so as to check the improvement in the quality of the communications. However, these operations remain constraining since on the one hand they require very high terrain experience from the person responsible for performing the analysis and on the other hand the disconnecting of the suspect base stations leads to a worsening of the service. Furthermore, in a highly urbanized configuration as is for example the case in a town such as Paris where the microcellular deployment is substantial, analysis based on experience is hardly to be envisaged any longer.
The purpose of the invention is to alleviate the aforementioned drawbacks by proposing a solution for the reliable determination of interference between traffic frequencies which do not intrinsically possess markers.
To this end, the subject of the invention is a process for analysing interference in a cellular radio communication system comprising beacon frequencies and traffic frequencies, characterized in that, in order to search on a specified analysis frequency and inside a cell for the origin of interference caused by neighbouring cells, it consists:
in performing a multipath synchronization on the learning sequences of the beacon frequencies so as to determine the number of base stations using the analysis frequency as beacon frequency,
in demodulating after spatial filtering the identifiers of the base stations transmitting the beacon frequencies on which a synchronization has been made,
in performing a multipath synchronization on the learning sequences of the traffic frequencies so as to determine the number of base stations using the analysis frequency as traffic frequency.
The subject of the invention is also a device for implementing the aforesaid process.